Lead is well known for poisoning paint-chewers and protecting radiographers' gonads. But if you cool a bit of lead to an uber-chilly - 265.8°C, it can suddenly do two very weird things.

For starters, it can suddenly repel magnets — pretty neat for something that's not usually magnetic. But the real action starts when you run a current through some supercooled lead wire — it conducts the current perfectly, not losing any energy as heat. There's zero resistance. More ridiculous still, if you shape the wire into a ring and then shut off the power source, the current would still keep flowing in the lead wire forever, never getting any weaker. Not bad for something that's a very ordinary conductor at regular temperatures.

It's called superconducting, and at - 265.8°C (7.2 Kelvin) or below, lead becomes a superconductor. And it's not alone. A lot of other metals and metal oxides do exactly the same trick, and they've each got their own critical temperature where the ridiculous behaviour kicks in — mostly below - 263°C (10 K).

Inside every one of them the same thing is happening: their loose electrons suddenly pair up and start shooting through the material without ever banging into any of the zillions of atoms jammed in there. These electron pairs are suddenly untouchable. And because they don't ever bump into anything, they don't lose any energy, and they don't create any heat. They're like a subatomic version of perpetual motion — as long as they're kept super-cold.

Superconduction sounds a bit far-fetched, and it happens instantly as the temperature drops, so it should come as no surprise that it's the result of a bit of quantum action.

Super-cool, superconducting shenanigans

Like all solids, lead is made up of neatly packed atoms jammed close together. And like all metals, lead atoms are a bit rubbish at holding onto their outermost electrons. So lead is really a bunch of positive lead ions soaking in a bath of loose electrons. (Read more about the structure of metals.). It's those loose electrons that conduct heat and electricity in all metals, but to be a superconductor you've got to have the right mix of loose electrons and flexible atomic packing.

At normal temperatures, the positive lead ions vibrate away on the spot and constantly collide with the electron bath around them. It is these collisions that cause the electrical resistance that wastes energy and produces heat in any normal circuit. But the cooler the solid gets, the less energy the ions have, so the less they vibrate.

When lead reaches its critical temperature, the ions' vibrations are incredibly weak and no longer the dominant form of motion in the lattice. The tiny attractive force of passing electrons that's always been there is suddenly enough to drag the positive ions out of position towards them. And that dragging affects the behaviour of the solid as a whole.

When positive ions are drawn towards a passing electron, they create an area that's more positive than their surroundings, so another nearby electron is drawn towards them. Those electrons are on the move though, so by the time the second one has arrived the first one has moved on and created a path of higher positivity that the second electron keeps on following. They're hitched in a game of catch-up that last as long as the temperature stays low.

These electron duos are called Cooper pairs, and they're what's behind the crazy sounding behaviour of superconductors.

Cooper did some serious slog to earn those naming rights. He and his mates Bardeen and Schrieffer came up with the mathematical theory that explains what's going on in all low-temperature superconductors, like lead, aluminium, mercury and hundreds more with way less reader-friendly names.

The Bardeen Cooper Schrieffer (BCS) theory explains not just electron pairing, but the tricky bit that stops them from ramming into all those positive ions suddenly leaning their way. Of course, it does it in a way that's incomprehensible to anyone short of a quantum physicist, so here's the physics-lite explanation:

At the critical temperature for superconducting (it's a different temperature for different materials), the loose electrons don't just pair up, all of those paired electrons become part of one big, electron superfluid. It's called a condensate, and it's got incredibly low energy.

The thing with this superfluid state is that the Cooper pairs that make it up are now totally co-dependent. One pair can't act without all the other pairs doing the same thing. So you can't have just one Cooper pair interacting with (ie smashing into) any of the positive ions that make up the structure — if one pair bangs into the lattice, that changes the energy of all the other pairs in the bath. And at those incredibly low temperatures there's not nearly enough energy for that collective interaction to happen.

So as long as the temperature stays at, or below, the critical point for that material, all of the loose Cooper-paired electrons will carry current without ever hitting the positive ion lattice, so there is exactly zero resistance to the flow of current. No resistance means no waste heat, and no waste heat means a much greater current can be run through the material than at normal temperatures. And huge currents make for hugely powerful electromagnets — which is the main application of superconductors.

Superconducting electromagnets are the expensive part of the MRIs used in medicine, and they control the beams in particle accelerators like synchrotrons and the Large Hadron Collider. And they've been used experimentally to soup up magnetic levitation (maglev) trains, and in components that could wind up in quantum computers. The magnetic fields from these superconducting electromagnets are so strong they can bring out magnetic characteristics in things not traditionally associated with attracting iron filings — frogs, grasshoppers and strawberries have all been levitated by them!

But their performance as electromagnets has nothing to do with the magnet-repelling trick superconductors master the second they hit their super-cool stride. It's called the Meissner effect, and it's the highlight of any halfway decent physics party trick.

If you put a magnet above a superconductor, the magnet floats. Its magnetic field generates currents in the surface of the superconductor, and those resistance-free currents (carried by Cooper pairs) generate their own magnetic field that pushes out the magnet's field and repels the magnet. Throw in a couple of tigers and you've got a show worthy of Vegas.

If I was writing this article in 1985, the story would end right here. But right about the time that shoulder pads and quiffs totally maxed out, some physicists blew away a part of BCS theory when they found ceramics that could superconduct at much higher temperatures than metals. BCS had set an upper temperature limit of 30 K (- 243°C) for superconducting, but these guys could practically levitate frogs at temperatures twice that high.

A couple of decades later we've got a bunch of different ceramics (mostly based on copper) that can superconduct at temperatures as balmy as - 123°C (150 K).

It was forty years before B, C and S nutted out what was going on in the low temperature superconductors, and in the 25 years since that ceramic breakthrough no one has figured out what makes these high temperature guys tick. But more important is getting to the next level — finding materials that superconduct at really high temperatures, something approaching our workaday fridge/freezer jobs. Without the need for all those expensive coolants (liquid helium or liquid nitrogen) superconductors suddenly become affordable, and that could totally revolutionise our energy and transport systems.

There have been lots of claims of success for super-high-temperature superconductors, but no clear evidence so far. So if someone offers to sell you one, make sure they can make a frog float with it at room temperature before you part with any cash.